1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a multi-domain liquid crystal display device that is capable of forming a multi-domain using a gate line.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) device controls a light transmissivity of liquid crystal cells in accordance with a video signal to display a picture. Such a LCD device has an advantage of small dimension, thin thickness and low power consumption while having a disadvantage of narrow view angle. An active matrix type LCD provided with a switching device for each liquid crystal cell is adaptive for displaying a moving picture. In the active matrix LCD, a thin film transistor (TFT) has been largely used as the switching device.
The active matrix LCD displays a picture corresponding to a video signal such as television signal on picture elements or pixels arranged at each intersection between gate lines and data lines. Each pixel includes a liquid crystal cell controlling a transmitted light amount in accordance with a voltage level of a data signal from the data line. The thin film transistors are formed at intersections between the gate lines and the data lines to switch a data signal to be transferred into the liquid crystal cell in response to a scanning signal from the gate lines.
Recently, there has been suggested a scheme of adjusting an orientation of the liquid crystal cells in a different direction at each of sub-pixels or domain divided into more than two within one pixel in order to compensate for a narrow view angle of the LCD. Such a LCD includes a multi-domain LCD device in which side electrode lines are provided around the pixel electrodes. This multi-domain LCD device drives a liquid crystal by the side electrodes insulated electrically from the pixel electrodes without orientating the liquid crystal cells.
FIG. 1 is a section view showing a structure of a unit pixel included in the conventional multi-domain LCD device. In FIG. 1, the unit pixel includes first and second substrates, a plurality of data lines and gate lines arranged horizontally and vertically on the first substrate to divide the first substrate into a plurality of pixel areas, and a thin film transistor at each pixel area on the first substrate. The thin film transistor (TFT) comprising of a gate electrode, a gate insulating film, a semiconductor layer, an ohmic contact layer and a source/drain electrode, a protective film 26 formed on the entire first substrate, a pixel electrode 28 provided on the protective film 26 to be connected to the drain electrode, and side electrodes 24 provided on the gate insulating film in such a manner to be overlapped with a portion of the pixel electrode 28. The unit pixel further includes a black matrix 30 provided on the second substrate to shut off a light leaked from the gate line, the data line and the thin film transistor, a color filter 32 provided between the black matrices 30 in correspondence with the pixel area, a common electrode 34 provided on the black matrix 30 and the color filter 32, and a liquid crystal layer 36 between the first and second substrates. The side electrodes 24 in the neighborhood of the pixel and an open area 35 of the common electrode 34 distort an electric field applied to the liquid crystal layer 36 to drive liquid crystal molecules diversely within the unit pixel. In other words, when a voltage is applied to the LCD device, a dielectric energy caused by the distorted electric field positions a liquid crystal director in a desired direction. In this case, the LCD device requires the open area 35 at the common electrode so as to obtain a multi-domain effect. A degree of the electrical field distortion required for a domain division is weak when the open area 35 does not exist in the common electrode 34 or when a width of the open area is small, a time when the liquid crystal director arrives at a stable state is relatively lengthened.
In the LCD device, however, because the side electrodes 24 taking a shape of surrounding the circumference of the pixel electrode 28 are used, an aperture ratio proportional to a size of the pixel electrode 28 is reduced. Accordingly, a brightness of the LCD device is deteriorated. Also, in FIG. 1, the side electrodes 24 are formed on the same layer as the data lines. In this case, the data lines are liable to be shorted to the side electrodes 24, and a line coupling in the data direction is generated, if shorted. In order to overcome this problem, it is necessary to assure a sufficient distance between the data line and the side electrode 24.
Accordingly, a size of the pixel electrode 28 goes smaller to further reduce an aperture ratio. In addition, the conventional side electrode 24 has a drawback in that, since it is formed in a line type and its width is set to have a value as small as possible (e.g., 6 μm) in consideration of an aperture ratio, that is, a size of the pixel electrode 26, it has a large resistance value. As a resistance value of the side electrode 24 is large, a voltage deviation caused by a resistance component thereof increases at the side electrode 24 when applied to a large-dimension panel. Particularly, a common voltage is applied to the side electrode 24 from each side of the panel, and a resistance value of the side electrode 24 is more and more increased to enlarge a voltage deviation as it goes toward the innermost side of the panel. Accordingly, as a potential difference between the pixel electrode 28 and the side electrode 24 is differentiated for each liquid crystal cell, the brightness becomes non-uniform and a flicker and a residual image, etc. are generated to thereby cause a deterioration of picture.
FIG. 2 is a section view showing the structure of a multi-domain LCD device disclosed in the pending Korea Application No. 99-05587 filed by the same applicant. In FIG. 2, the multi-domain LCD device includes thin film transistors (TFT's) 6 arranged at each intersection between data lines 2 and gate lines 4, pixel electrodes 14 connected to drain electrodes 10 of the TFT's 6, and auxiliary electrode lines 16 provided at the circumferences of the pixel electrodes 14. Each TFT 6 comprises a gate electrode 12 connected to the gate line 4, a source electrode 8 connected to the data line 2, and a drain electrode 10 connected, via a drain contact 11, to the pixel electrode 14. The TFT 6 further includes a semiconductor (not shown) for providing a channel between the source electrode 8 and the drain electrode 10 with the aid of a gate voltage applied to the gate electrode 12. Such a TFT 6 responds to a gate signal from the gate line 4 to selectively apply a data signal from the data line 2 to the pixel electrode 14. The pixel electrode 14 is formed at a cell area divided by the data line 2 and the gate line 4, and comprises an electrode made from an indium tin oxide (ITO) material having a high light transmissivity. This pixel electrode 14 generates a potential difference from a transparent electrode (not shown) formed on an upper glass substrate by a data signal applied via the drain contact 11. At this time, the liquid crystal is rotated by its dielectric anisotrophic property to transmit a light supplied, via the pixel electrode 14, from a light source toward the upper substrate. The auxiliary electrode line 16 generates a potential difference from the pixel electrode 14 in a scanning interval when a data signal is applied to the liquid crystal cell to adjust an orientation of the liquid crystal, thereby forming a multi-domain. In this case, a common voltage Vcom is applied from an external common voltage generator to the auxiliary electrode line 16. A boundary line 15 of a matrix formed on the upper substrate is located on the auxiliary electrode line 16 in such a manner that the matrix covers all portions of the auxiliary electrode line 16.
In the above-mentioned LCD device, however, because the auxiliary electrode lines 16 taking a shape of surrounding the circumference of the pixel electrode 28 are used likewise, an aperture ratio proportional to a size of the pixel electrode 14 is reduced. Accordingly, a brightness of the LCD device is deteriorated. The auxiliary electrode lines 16 are usually formed on the same layer as the gate lines 4 as shown in FIG. 3. In this case, the gate lines 4 are liable to be shorted to the auxiliary electrode lines 16, and a line coupling in the data direction is generated, if shorted. In order to overcome this problem, it is necessary to assure a sufficient distance d between the gate line 4 and the auxiliary electrode 16. Accordingly, a size of the pixel electrode 14 goes smaller to further reduce an aperture ratio. In addition, the auxiliary electrode line 16 has a drawback in that, since it is formed in a line type and its width is set to have a value as small as possible (e.g., 6 μm) in consideration of an aperture ratio, that is, a size of the pixel electrode line 14, it has a large resistance value. As a resistance value of the auxiliary electrode line 16 is large, a voltage deviation caused by a resistance component thereof increases at the auxiliary electrode line 16 when applied to a large-dimension panel. Particularly, a common voltage is applied to the accompanying electrode line 16 from each side of the panel, and a resistance value of the auxiliary electrode line 16 is more and more increased to enlarge a voltage deviation as it goes toward the innermost side of the panel. Accordingly, as a potential difference between the pixel electrode 14 and the auxiliary electrode line 16 is differentiated for each liquid crystal cell, the brightness becomes non-uniform and a flicker and a residual image, etc. are generated to thereby cause a deterioration of picture.